seen in Figure 8, the maximum dissolution efficiency was obtained at 70 oC.
It was observed that colemanite was completely dissolved at 70 °C in this study.
acid crystallization starts below 70 oC. Thus when the dissolution solution from a reactor is
transported in pipes, the boric acid solution temperature will decrease in the
pipes and filters and the boric acid solution will crystallize, so that the
pipes and filters will be clogged by these crystals. For this reason,
industrial dissolution of concentrated colemanite with sulfuric acid is
performed at 85 ± 3 °C.
of leaching and crystallization products
leaching process of CC in sulfuric acid solution takes place via the following
set of reactions. The dissolution of sulfuric acid is obtained in an aqueous
medium as follows:
overall reaction is as follows:
reaction of colemanite with sulfuric acid results in the products gypsum and
boric acid. The dissolution of calcite is obtained as follows:
reaction between calcite and sulfuric acid results in gypsum.
optimum conditions were found to be 1.0 mol/L sulfuric acid concentrations due
to high acid concentration requiring for colemanite dissolution and gypsum
formation, a 500 rpm stirring speed, a 50 g/L solid to liquid ratio and a
solution temperature of 70 °C. 50 g/L
solid CC was added to the solution prepared for optimum conditions and was leached for 30 minutes. The leach solution
was filtered with filter paper. The liquid phase was
crystallized in a water bath at a temperature of 35 oC to obtain
boric acid crystals. The liquid phase was not dried at ambient temperature due
to the hygroscopic nature of gypsum. Instead, it was dried in an oven at 110 oC
for XRD analysis. As seen in Figure 9, anhydrite (gypsum) and sassolite (boric
acid) were observed in the XRD analysis.
Fig. 9. X-ray diffraction analysis of crystallized
liquid phase. (acid concentration of 1.0 mol/L, solid to liquid ratio of 50
g/L, speed of 500 rpm, for 30 minutes).
solid phase was dried at 110 oC, the crystal water evaporated from
the gypsum, which became anhydrous because of its low dehydration temperature. The CC was leached using a sulfuric acid
solution at the aforementioned optimum conditions for 120 minutes. The leachant
solutions were filtered, and the liquid phase was crystallized at 35 oC.
The crystals obtained were dried at ambient temperature due to the low
dehydration temperature of boric acid, and the crystallized liquid phase was
analyzed by XRD (Rigaku D/Max-2200/PC).
Fig. 10. X-ray diffraction analysis of crystallized
liquid phase for 120 min. (acid concentration of 1.0 mol/L, solid to liquid
ratio of 50 g/L, speed of 500 rpm, for 120 minutes).
shows the XRD patterns of the crystals. Only boric acid (H3BO3)
peaks were observed by XRD analysis. The result of XRD is in agreement with
equation (3). The colemanite and
sulfuric acid reaction equation (3) resulted in boric acid.
An SEM (JSM–6060 JEOL)
image of crystalline boric acid is given in Figure 11. The boric acid
crystals, which have a triclinic structure, can be observed in the SEM images. The XRD and SEM analyses show that pure boric acid crystals can be
produced using CC. As a result of chemical analysis, the solid residue
was found to contain 30.2% CaO and % 35.11 % SO3. The coarse
particle size of gypsum did not allow it to pass the filtration media with the